Which of the following $3d$-metal ion will give the lowest enthalpy of hydration $(\Delta_{hyd}H)$ when dissolved in water?

Compare the general characteristics of the first series of the transition metals with those of the second and third series metals in the respective vertical columns. Give special emphasis on the following points:
$(i)$ electronic configurations,
$(ii)$ oxidation states,
$(iii)$ ionisation enthalpies,and
$(iv)$ atomic sizes.

Which of the following metals exhibits more than one oxidation state?

Which one of the following does not correctly represent the correct order of the property indicated against it?

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: In $T \ell I_{3}$,isomorphous to $CsI_{3}$,the metal is present in $+1$ oxidation state.
Reason $R$: $T \ell$ metal has fourteen $f$ electrons in the electronic configuration.
In the light of the above statements,choose the most appropriate answer from the options given below:

Which of the following pairs of elements have one electron in the $5d$-subshell in their observed electronic configuration?